Touch panels are used in various applications to replace conventional mechanical switches: e.g., kitchen stove, microwave ovens, and the like. Unlike mechanical switches, touch panels contain no moving parts to break or wear out. Mechanical switches used with a substrate require some type of opening through the substrate for mounting the switch. These openings, as well as openings in the switch itself, allow dirt, water and other contaminants to pass through the substrate or become trapped within the switch. Certain environments contain a large number of contaminants which can pass through substrate openings, causing electrical shorting or damage to the components behind the substrate. However, touch panels can be formed on a continuous substrate sheet without any openings in the substrate. Also, touch panels are easily cleaned due to the lack of openings and cavities which collect dirt and other contaminants.
Existing touch panel designs provide touch pad electrodes attached to both sides of the substrate; i.e., on both the"front" surface of the substrate and the "back" surface of the substrate. Typically, a tin antimony oxide (TAO) electrode is attached to the front surface of the substrate and additional electrodes are attached to the back surface. The touch pad is activated when a user contacts the TAO electrode. Such a design exposes the TAO electrode to damage by scratching, cleaning solvents, and abrasive cleaning pads. Furthermore, the TAO electrode adds cost and complexity to the touch panel.
Known touch, panels often use a high impedance design which may cause the touch panel to malfunction when contaminants such as water or other liquids are present on the substrate. This presents a problem in areas where liquids are commonly found, such as a kitchen. Since the pads have a higher impedance than the water, the water acts as a conductor for the electric fields created by the touch pads. Thus, the electric fields follow the path of least resistance; i.e., the water. Also, due to the high impedance design, static electricity can cause the touch panel to malfunction. The static electricity is prevented from quickly dissipating because of the high touch pad impedance.
Existing touch panel designs also suffer from problems associated with crosstalk between adjacent touch pads. The crosstalk occurs when the electric field created by one touch pad interferes with the field created by an adjacent touch pad, resulting in an erroneous activation such as activating the wrong touch pad or activating two pads simultaneously.
Known touch panel designs provide individual pads which are passive. No active components are located in close proximity to the touch pads. Instead, lead lines connect each passive touch pad to the active detection circuitry. The touch pad lead lines have different lengths depending on the location of the touch pad with respect to the detection circuitry. Also, the lead lines have different shapes depending on the routing path of the line. The differences in lead line length and shape cause the signal level on each line to be attenuated to a different level. For example, a long lead line with many corners may attenuate the detection signal significantly more than a short lead line with few corners. Therefore, the signal received by the detection circuitry varies considerably from one pad to the next. Consequently, the detection circuitry must be designed to compensate for large differences in signal level.
Many existing touch panels use a grounding mechanism, such as a grounding ring, in close proximity to each touch pad. These grounding mechanisms represent additional elements which must be positioned and attached near each touch pad, thereby adding complexity to the touch panel. Furthermore, certain grounding mechanisms require a different configuration for each individual touch pad to minimize the difference in signal levels presented to the detection circuitry. Therefore, additional design time is required to design the various grounding mechanisms.
The use of conventional touch panels or touch sensors in stoves, microwave ovens, and the like, places such touch sensors in an environment where they can potentially come into frequent contact with conductive liquids or contaminants. The presence of a conductive liquid on any touch sensor could create a false output thereby causing the control circuit to initiate an output action where none was intended. Such liquids, when in the form of a large puddle or drops, can actually span two or more individual touch sensors. This again leads to the potential for false input signals.
Recent improvements in touch panel design include techniques which lower the input and output impedance of the touch sensor itself, thereby making the sensors highly immune to contaminants and false activations due to external noise sources. U.S. Pat. No. 5,594,222 describes such a technique. Even though this approach has several advantages over the prior art, there are some attributes that may limit its application. For instance, the resulting sensor may be inherently sensitive to temperature variations. As long as the temperature variations at the output are small relative to legitimate signal changes and are small relative to signal variations due to transistor variations, then a single transistor or other amplifying device will be quite satisfactory. However, in applications where there is little dynamic range to allow for compensation by software and where temperature changes are significant relative to legitimate signal changes, another approach would be useful to eliminate or greatly reduce the effects of temperature. Also, even though the low impedance approach of this technique can differentiate between contaminants with some finite amount of impedance and a human touch with some finite amount of impedance, this technique may not be enough to inherently differentiate extremely low levels of impedance. Such examples of this situation would exist when a sensor (i.e., both the inner and outer electrode) is covered with a large amount of contaminants, greatly reducing the impedance of the inner pad. Another example would be where a conductive material such as a metal pan covers an entire singular sensor.
Thus, it would be desirable to provide a touch panel which prevents false signal generation in the presence of highly conductive materials, relatively substantial temperature changes, and other effects common to both the inner and outer electrode and associated circuitry.